In recent years, there is an increasingly growing demand of reliability for semiconductor devices, and in particular, there is a strong demand to improve reliability for a bonding portion between a semiconductor element and a circuit board having a large difference in thermal expansion coefficient therebetween. Heretofore, semiconductor elements whose base materials are silicon (Si) and gallium arsenide (GaAs) have been often used, and their operation temperatures are from 100° C. to 125° C. As soldering materials for bonding the semiconductor elements to electrodes of electronic circuits, there are used 95Pb-5Sn (mass %) for Si devices, 80Au-20Sn (mass %) for gallium arsenide devices, and the like, from viewpoints of: crack resistance against repetitive thermal stress due to difference in thermal expansion between a semiconductor element and a circuit board; high melting point to meet a multistage solder-bonding at the time of assembly; and further, contamination tolerance of the devices. However, from the aspect of reducing environmental load, 95Pb-5Sn containing a large amount of harmful lead (Pb) is problematic, and further, from a viewpoint of price-rise and reserve of novel metals, a substituent material for 80Au-20Sn is strongly demanded.
On the other hand, from the aspect of saving energy, devices whose base materials are silicon carbide (SiC) and gallium nitride (GaN) have been under active development as next-generation devices. From the aspect of reducing power loss, they are required to have operation temperatures of 175° C. or more and it is said that the temperature will become 300° C. in future.
For dealing with the above requirement, a high-temperature soldering material (high-temperature solder alloy) is required that has a superior thermal resistance as well as a high melting point. Such a solder alloy has been hitherto a Pb-based solder alloy having a melting temperature of around 300° C. Its examples include Pb-10Sn (mass %), Pb-5Sn (mass %), Pb-2Ag-8Sn (mass %), Pb-5Ag (mass %) and the like, and hence, Pb is mainly given as a major component. The solidus temperature of Pb-10Sn is 268° C. and its liquidus temperature is 302° C. The solidus temperature of Pb-5Sn is 307° C. and its liquidus temperature is 313° C. The solidus temperature of Pb-2Ag-8Sn is 275° C. and its liquidus temperature is 346° C. The solidus temperature of Pb-5Ag is 304° C. and its liquidus temperature is 365° C.
Meanwhile, from the aspect of environmental protection, it has recently been required generally in soldering technology, to use a Pb-free solder alloy instead of the Pb series solder alloy. As a matter of course, with respect to the aforementioned Pb—Sn series high-temperature solder having been used for conventional semiconductor devices, it has also been required to instead use a Pb-free solder alloy.
However, while a variety of Pb-free solder alloys have been proposed hitherto, they consist mainly of Sn, so that there is no high-temperature solder alloy whose solidus temperature is 260° C. or more. For example, in an Sn—Ag series solder alloy whose solidus temperature (eutectic temperature) is 221° C., as Ag is increased, the liquidus temperature rises but the solidus temperature does not rise. In an Sn—Sb series solder alloy whose solidus temperature is 227° C., if Sb is extremely increased in order to make the solidus temperature higher, the liquidus temperature also becomes higher extremely. Further, it is unable to change such properties even if another element is added to them. Thus, it is conventionally thought that there is no Pb-free solder alloy that does not melt even at 300° C. and is thus usable as a solder.
For that reason, a bonding technology without using a high-temperature solder alloy has been under consideration. What has been considered as the bonding technology without using a high-temperature solder alloy is bonding methods by use of an intermetallic compound having a melting temperature higher than that of the Pb-free solder consisting mainly of Sn. Of these, in particular, a bonding method by means of an intermetallic compound of Ag and Sn (Ag3Sn) is promising in which Ag is used that diffuses quickly into Sn to thereby form the intermetallic compound at a relatively low temperature.
For example, in Patent Document 1, there is described a composite solder that is Pb-free and can be used for high-temperature-side solder bonding in a temperature-hierarchical bonding. In Patent Document 1, the composite solder has a configuration in which a metal net made of Cu is sandwiched and pressure-bonded between two solder foils, and such a fact is shown that when the metal net and the solder foils are thus-stacked and press-formed together, Sn of the solder foils is penetrated into apertures of the metal net, so that an intermetallic compound of Cu and Sn (Cu3Sn, Cu6Sn5) is formed after heating to thereby achieve enhancement in thermal resistance. Further, in Patent Document 1, there is shown that a net of Ag other than Cu is likewise an important candidate, and an Ag3Sn compound that is a high-melting-point intermetallic compound allows a joint connection that doesn't melt even at 280° C. There is also shown that, as another alloy series that is hard and has a low melting point like the above, a Cu—Sn series (for example, Cu6Sn5) can accommodate in a similar manner.
As another instance, in Patent Document 2, there is described a bonding sheet for bonding a chip (semiconductor element) and a die together. The bonding sheet in Patent Document 2 comprises an Ag sheet with shaped grooves or a mesh-like sheet by warp and weft knitting of Ag wires. An Sn plating of a thickness of 0.3-2.0 μm is applied on the surface of the Ag sheet, so that when subjected to pressing and heating, Ag is supplied thereto successively from the Ag sheet as a core due to melting or diffusion at the time of heating. Thus, the bonding sheet in Patent Document 2 is shown as being capable of raising the melting point of a finally formed Ag—Sn layer to 470° C. or more, to thereby provide a highly thermal-resistant bonding portion. Further shown is that the Ag sheet having in-groove spaces is so soft as to absorb thermal distortion, to thereby enhance reliability.